Historically, end mills for metal cutting machinery were produced as a single unit, comprising a fluted cutting portion and a cylindrical or conical shank portion sized to fit a machine spindle. However the increasing global pricing of modern tool alloys along with recently developed intricate surface treatment, made it less economical, as the expensive shank material was wasted. It is therefore became common practice to produce a separate cutter made of high quality alloy or sintered carbide, which is then concentrically attached to the end of a reusable steel shank.
It is highly desirable that the cutter be easily replaced, upon wear, while leaving the shank in the machine spindle, such that no further adjustments are required after cutter replacement. A major requirement related to such accurate milling applications is that each replacement cutter be repeatedly, accurately, centered to the true spindle axis of rotation and axially positioned correctly.
One basic method currently in use for joining the cutter to the shank is disclosed for example in U.S. Pat. No. 5,114,286, teaching an interchangeable cutting tool alignment and positioning system comprising a first tool segment having a male coupler and a second tool segment having a female coupler. The male coupler comprises a pilot in the form of first cylindrical mating surface, a concentric aligner in the form of second cylindrical mating surface spaced apart from the pilot, a male thread extending between the pilot and the concentric aligner and an axial stop in the form of planar surface. The female coupler comprises a pilot bore in the form of complementary cylindrical mating surface, corresponding to the cylindrical mating surface of the pilot, a concentric bore in the form of a complementary cylindrical mating surface corresponding to the cylindrical mating surface of the concentric aligner, a female thread extending between the pilot bore and the concentric bore, and an axial stop in the form of complementary planar surface.
The described pilot, concentric aligner, pilot bore and concentric bore, are necessary because the threaded coupler by its own is not sufficiently accurate for such repeated replacement of cutters.
Further improvements to the above basic concept are also known. For instance, U.S. Pat. No. 6,485,220 discloses a frustoconical radial alignment instead of a cylindrical alignment, as well as a strengthened thread root. U.S. Pat. No. 7,329,073 describes adjacent axial and radial stop surfaces, while US 2007/011653 discloses self centering multi-start thread.
Nevertheless all the above described solutions suffer from restrictive production requirements. Typical production tolerances of the cylindrical mating surfaces on the cutter and shank, sufficient for satisfying the need of replaceable cutters falling repeatedly in the desired range of concentricity and axis position, are less than 5 micrometers. Such close tolerances necessitate an additional grinding process.
Furthermore, sintered carbide cutters by their nature are very hard yet also very brittle. Direct coupling of the hard cutter to the steel shank imposes repeated vibration and shock on the cutter produced during the milling operation. If this shock is not properly absorbed, it will end with fracture and early failure of the tool.
Additionally, by virtue of fit tolerances i.e. the coincidence of mutually contacting parts, and their statistical dispersion, in some cases, there will be a gap of up to 10 micrometers between the shank bore and the cutter mating surface. The presence of such a gap, besides the resulting run-out itself, may also, when subjected to the afore-mentioned vibration, loosen the thread coupler during machine operation, typically a disastrous situation.
Consequently, an improved, quick-change joint for coupling a replaceable round tip to a reusable shank is required, that will eliminate the need of tight tolerances as well as absorb undesired vibration and shock while reducing the risk of fracture and unexpected joint slack.